Use Mimir version referenced in Helm charts documentation

Signed-off-by: Jack Baldry <[email protected]>

.github/workflows/deploy-pr-preview.yml

@@ -12,6 +12,8 @@ on:
 jobs:
   deploy-pr-preview:
     if: "!github.event.pull_request.head.repo.fork"
+    env:
+      MIMIR_VERSION: v2.15.x
     uses: grafana/writers-toolkit/.github/workflows/deploy-preview.yml@jdb/deploy-preview
     with:
       branch: ${{ github.head_ref }}
@@ -21,11 +23,11 @@ jobs:
       sources: |
         [
           {
-            "index_file": "content/docs/mimir/_index.md",
-            "relative_prefix": "/docs/mimir/latest/",
+            "index_file": null,
+            "relative_prefix": "/docs/mimir/${{ env.MIMIR_VERSION }}/",
             "repo": "mimir",
             "source_directory": "docs/sources/mimir",
-            "website_directory": "content/docs/mimir/latest"
+            "website_directory": "content/docs/mimir/${{ env.MIMIR_VERSION }}"
           },
           {
             "index_file": "content/docs/helm-charts/mimir-distributed/_index.md",